The present invention relates to a rare earth oxide-based garnet single crystal for magnetostatic device and a method for the preparation thereof. More particularly, the invention relates to a rare earth oxide-based garnet single crystal for magnetostatic device used in the microwave region such as filters, resonators, oscillator elements, signal-noise enhancers, isolators, circulators and the like and a method for the preparation of such a single crystal as well as to a magnetostatic device constituted by using the same.
As to the rare earth oxide-based garnet single crystal for magnetostatic devices, proposals have been made in the prior art for those having an epitaxially grown film of YIG (yttrium iron garnet) on the surface of a substrate garnet single crystal. Magnetostatic devices having such a garnet element have a defect that the temperature dependence of the saturation magnetization 4.pi.Ms of the YIG film is so large that the magnetic field applied to the YIG film must be varied in a wide range in order to decrease the temperature dependence of the working frequency of the device while such a means can be undertaken only with an elaborate instrumentation for the temperature compensation of the magnetic field consequently resulting in a great increase of the overall costs of the magnetostatic device. It is accordingly desirable to decrease the temperature dependence of the YIG film per se so low that no means for temperature compensation of the magnetic field is necessitated any longer to provide a magnetostatic device of good temperature characteristics at low costs.
In this regard, various proposals and attempts have been made heretofore for decreasing the temperature dependence of YIG films. For example, H. L. Glass, et al. disclose in Material Research Bulletin, volume 12, pages 735 to 740 (1977), that the vertical resonance magnetic field of a garnet film having a composition expressed by the formula of La.sub.0.06 Y.sub.2.34 Fe.sub.4.73 Ga.sub.0.87 O.sub.12 with a saturation magnetization of 410 G exhibits a quadratic temperature dependence assuming a constant frequency and P. Roschmann, et al. disclose in Material Research Bulletin, volume 18, pages 449 to 459 (1983), that a garnet of the formula YSFe4.07Ga0.93012 exhibits a quadratic temperature dependence of the ferromagnetic resonance frequency at or in the vicinity of room temperature assuming a constant magnetic field to have a relatively good stability of the resonance frequency against temperature changes.
The principal mechanism of the above mentioned temperature compensation effects can be described in terms of the Kittel's resonance equation relative to the vertical resonance in the direction of &lt;111&gt;: EQU f=r.multidot.(Hres-N.multidot.4.pi.Ms-4/3.multidot.K1/Ms), (I)
in which f is the frequency, r is the gyromagnetic ratio, 4.pi.Ms is the saturation magnetization, Hres is the vertical resonance magnetic field, N is the effective demagnetization factor or so-called garnet coefficient and K1 is the first-order anisotropy constant of a cubic crystal.
While the temperature compensation effect reported in the above mentioned literatures is presumably due to offsetting between the changes in the 4.pi.Ms and the changes in the K1 caused by the changes in temperature, the ranges of the changes in the resonance frequency or resonance magnetic field are reportedly only 10 MHz or 3 Gauss, respectively, corresponding to a temperature change of 40.degree. C. at or in the vicinity of room temperature not to give a practical means as a solution of the above mentioned problems.
It is reported by T. Ryuo et al. in Preprints of IEEE Ultrasonic Symposium (1988), pages 237 to 240, that a rapid degradation is caused in the magnetic resonance half value width .DELTA.H corresponding to 500 G or smaller 4.pi.Ms at room temperature in a YIG film substituted by lanthanum and gallium epitaxially grown on a substrate of a yttrium-substituted GGG (gadolinium gallium garnet) prepared by the liquid-phase epitaxial method and exhibiting the same degree of the temperature compensation effect as mentioned above. These situations imply that it would be a difficult matter to prepare a low-loss magnetostatic device from the above described temperature-compensating materials because of the insufficient temperature compensation and poor .DELTA.H value.
In order for a magnetostatic device to be practically used as a high-frequency signal processing device, it would be an essential requirement that the frequency change of a magnetostatic device should not exceed 10 MHz within a temperature range equal to or 40.degree. C. or even broader in the vicinity of room temperature corresponding to about 3 Gauss or smaller of the variation in the vertical resonance magnetic field. Further, the .DELTA.H value at 9.2 GHz must be 1.5 Oe or smaller in order to prepare a low-loss magnetostatic filter or a high-Q magnetostatic resonator. It is accordingly desired to provide a magnetostatic device suitable for these practical applications at low manufacturing costs.